The social media are becoming a major channel of online interactive participation, and local governments are seizing this opportunity to enhance citizen engagement in political and social affairs. This article analyses the various forms of social media used – that is, Twitter or Facebook – by citizens in their relations with Spanish local government, to determine which of these achieves the strongest degree of commitment. We also analyse the influence of various factors on this level of commitment. The results obtained show that Facebook is preferred to Twitter as a means of participating in local government issues. Other factors that are relevant to citizen engagement are the level of online transparency, mood, the level of activity in social media and the interactivity offered by the local government website. The findings of this study contribute significantly to understanding how citizen engagement is influenced by the type of social media adopted.